Plastic pressure vessel for a fluid and method of manufacturing such a vessel

ABSTRACT

A plastic pressure vessel comprises a first ( 1, 2 ) and a second part ( 2, 1 ) welded to each other. The first part ( 1 ) comprises a layer ( 11 ) of thermoplastic material having substantially continuous fibres. The second part comprises a layered structure having an first ( 20 ) and a second ( 22 ) layer of thermoplastic material having substantially continuous fibres separated by a central ( 21 ) layer comprising thermoplastic material. The layer having substantially continuous fibres ( 11 ) of the first part extends over a length (d) sandwiched in between and connected with the first ( 20 ) and second ( 22 ) layer of thermoplastic material having substantial continuous fibres of the second part. Preferably the second part is provided with a slit ( 24 ) and the first part with a fitting protrusion, preferably with a bevelled end ( 23 ).

DESCRIPTION OF PRIOR ART

The invention relates to a plastic pressure vessel for a fluidcomprising a first and a second part connected to each other, one of thefirst and second part being a main body or an end part. Within theframework of the invention the term ‘fluid’ comprises gasses as well asliquids.

The invention also relates to a method of manufacturing a plasticpressure vessel comprising a first and a second part, one of the firstand second part being a main body or an end part, comprising a methodstep in which the main body and the end part are connected to eachother.

Plastic pressure vessels are known. One of the main advantages of usingplastic materials over metals is formed by a reduction in weight.

Conventionally the plastic pressure vessels are made by means of blowmoulding or injection moulding.

Although such vessels are relatively easy to make, the pressure range inwhich they operate safely is limited.

In EP 0 461 979 a pressure vessel is shown of which the main bodycomprises an inner layer of a thermoplastic material and an outer layerof a plastic material further comprising fibres. The main body is madeby winding the layers one over the other on a mandrel.

The end part is made by means of injection moulding of a thermoplasticmaterial.

In EP 0 461 979 the end part and the main body are connected to eachother by using an electrical resistor through which a current is fed.The generated heat fuses the parts together.

The known method has as a drawback that the electrical resistor remainsin the pressure vessel, which adds to the weight and in time couldweaken the pressure vessels. From WO 92/20954 a pressure vessel is knowncomprised of two tank halves comprising fibre-reinforced plasticmaterials. The edges of the tank halves are cut under an angle and gluedtogether. Around the glued seam a reinforcing girdle of fibre-reinforcedplastic material is positioned.

Although these known vessels function relatively well they cannot safelywithstand relatively high pressures or if they do, the costs ofproviding such safety is very high.

There is therefore a need for providing a pressure vessel capable ofwithstanding high pressures while at the same time doing so in aneconomic manner and in a flexible manner.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a pressure vessel that canwithstand on average higher pressures, while the production process anddesign remains flexible and at ‘low cost’.

To this end the first part comprises at least one layer of thermoplasticmaterial having substantially continuous fibres, and the second partcomprises a layered structure having a first and a second layer ofthermoplastic material, at least one of said first and second layershaving substantially continuous fibres, said first and second layersbeing separated by a inner plastic layer, the layer having substantiallycontinuous fibres of the first part extending over a length sandwichedin between and connected with the first and second layer ofthermoplastic material of the second part.

The substantially continuous fibres (i.e. having a length longer than 25mm, but preferably longer than 50 mm) of the at least one layer of thefirst part and of the first and/or second layer of the second part formover a length a connection. The sandwich-connection between the threelayers, at least two of which comprising continuous fibres, provides fora strong bridge between the parts enabling a substantial increase instrength of the vessel especially against high pressures. Whereas inconventional blow moulded or injection moulded plastic pressure vesselspressure up to 20-30 bars are attainable before burst at reasonablecosts and weight, pressure vessels in accordance with the invention haveattained pressures well over 75 bar, without (greatly) increasing thecosts. The inherent strength of the connection between the layers in thepressure vessel in accordance with the invention removes or at leastreduces the need for providing a reinforcing girdle around the gluedseam as known from WO 92/20954, reducing the complexity and costs of theprocess and the pressure vessel as well as offering the possibility of areduction in weight. Preferably both of the first and second layers ofthe second part comprise substantially continuous fibres. Providingsubstantially continuous fibres in both of the first and second layersadds additional strength of the connection and thus to the pressurevessel, such in comparison to embodiments of the invention in which onlyone of the first and second layers is provided with substantiallycontinuous fibres. Said latter embodiments, however, are less costly andthus suited for embodiments in which relatively lower maximum pressuresare applicable but costs are a major consideration. Preferably the innerlayer of the second part comprises thermoplastic material. Provision ofthermoplastic material in the inner layer gives the possibility that thelayer of the first part is also connected to the inner layer providingfurther strength to the connection.

Preferably at least one of the first and second part is a cylindricallyformed main body of the pressure vessel, at both sides being providedwith end parts.

In these embodiments the length of the cylindrical body may be chosen,offering a flexibility in the design, i.e. the volume of the pressurevessel is not restricted. In this respect it is remarked that the designshown in WO 92/20954 must be made of two halves, the present inventionremoves this restriction, enabling a more flexible design.

Preferably in such preferred embodiments the end parts of the vessel areprovided with attachment means and have a higher maximum burstingpressure that the main body. This condition ensures that the main bodyon average is the most likely to fail. If the pressure vessel is tofail, safety is best guaranteed if the likelihood of loose parts isreduced.

Preferably the second part comprises a slit into which a protrusion ofthe layer comprising substantially continuous fibres of the first partextends. Providing a slit, rather than forcing the first part into thesecond part, enables a better and stronger connection to be obtained.

Preferably the protrusion is bevelled. The provision of such aprotrusion (which could be in the form of a V-shape) increases thestrength of the seam considerably.

Preferably the connection of the end part and the main body is formedsuch that the continuous fibres containing layer of the first partextend, seen in a direction parallel to the layers, over a lengthbetween 10-100, preferably between 20 and 50 mm in between the first andsecond layers of the second part (at least one of said layers comprisingsubstantially continuous fibres). Too small an extension increases therisk that the seam comprises weak spots. However, too wide an overlapincreases the risk that the innermost or outermost parts of the seam arenot well connected, reducing the strength of the seam. The indicatedamount of overlap of the fibres containing layers in the end part andthe main body adds to the strength of the seam of the vessel without thementioned negative possible side-effects. Preferably the length of thesubstantially continuous fibres in all relevant layers is at leastequal, preferably at least twice the overlap between the layers.Preferably the substantially continuous fibres are laid in both partsand all relevant layers in a criss-cross pattern, extending across theseam. Such a pattern increases the strength of the seam.

Preferably the first part comprises an inner layer of thermoplasticmaterial. Such an inner layer upon which the layer comprisingsubstantially continuous fibres is provided, increases the air tightnessof the vessel, as well as chemical resistance to the fluid contained inthe vessel. Preferably, however, this inner layer does not extend in theconnection between the first and second part.

Preferably the first part comprises an outer layer over the layercomprising substantially continuous fibres. This outer layer providesfor a finish of the first part, as well as UV resistance and/or chemicalresistance to the fluids of the environment. Preferably, however, thisouter layer does not extend in the connection between the first andsecond part.

It is further an object to provide for a fast, safe and economic methodfor connecting the main body and the end part, yet resulting in a strongbond between them.

To this end the end part and the main part are connected to each otherby friction welding, i.e. by placing the to be connected surfaces of theto be connected layers of the first and second part against each otherand moving the surfaces along each other so that heat is generated. Thiscan be done by rotating (in case the contact area is circular) the mainbody and/or the end part around the axis of symmetry of the contactarea.

Within the framework of the invention ‘friction welding’ is to beunderstood as methods in which kinetic energy of one part is transferredinto heat by friction between the parts thereby enabling welding of thelayers This can be done in several ways:

-   -   One of the parts (main body or end part for instance) is spun,        whereafter the spun part is left to freely rotate, the parts are        brought close to each other, and the connection is made. The        friction between the freely rotating part and the stationary        part, along the overlap, provides heat, which melts the        thermoplastic material and provides for a connection. This may        be called ‘rotational inertia friction welding’, i.e. the        inertia of the rotation part provides for the heat, via        friction. Instead of letting the rotation part freely rotate,        said part is rotated during the welding operation. This has the        advantage of a better control over the supply of heat. This        could be called ‘forced rotational friction welding’    -   One of the parts is not rotated but vibrated. Such vibrational        movement can be back and forth along the overlap, or a        vibrational rotational movement, i.e. a twisting movement, or        any combination of such movements.

The fact that the fibres are substantially continuous adds to thestrength of the connection between the end part and the main body.

Preferably the vessel comprises a central body part and two end parts,the central part and the end parts as second parts are friction weldedtogether while the end parts and/or the main body are rotated.Simultaneous welding of the central and end parts together improves thespeed of the process. Preferably the end parts are rotatedsynchronously. Preferably the end parts are provided with handlesintegrated, prior to friction welding. Such handles can be used toconnect the pressure vessel to other vessels or to a supportingstructure. (f.i. the frame of a truck). Providing the handles afterfriction welding is more complicated and costly. The synchronousrotation of the end parts ensures that the relative position of thehandles stays the same. ‘Handle’ is within the concept of the inventionto be understood any element to which something may be attached orconnected.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereafter.

SHORT DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows schematically a main body and an end part of a pressurevessel in accordance with the invention.

FIG. 2 shows schematically a connection between a main part and an endpart of a pressure vessel in accordance with the invention.

FIG. 3 shows schematically a connection between a main part and an endpart of a pressure vessel in accordance with a preferred embodiment ofthe invention.

FIG. 4 shows schematically how the layers are connected together

FIG. 5 illustrates a preferred embodiment for manufacturing an end cap

FIG. 6 illustrates the method in accordance with the invention.

The figures are not drawn to scale. Generally, identical components aredenoted by the same reference numerals in the figures. All figures aregiven by means of illustration of exemplary embodiments of theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a main body 1 of the pressure vessel. The main bodycomprises in this exemplary embodiment a layered structure whichcomprises an outer layer 10 of a thermoplastic material, a central layer11 of a thermoplastic material comprising substantially continuousfibres and an inner layer 12 of thermoplastic material. The inner layer12 and the outer layer 10 are within the framework of the inventionoptional but preferred layers, the inner layer 12 can be useful toincrease the air-tightness of the vessel and chemical resistance, whilethe outer layer 10 can be useful to improve the visual appearance of thevessel and UV and/or chemical resistance. The end part 2 comprises anouter layer 20 of thermoplastic material comprising substantiallycontinuous fibres, an inner layer 22 comprising substantially continuousfibres and a central layer 21 comprising in this example a thermoplasticmaterial. The thermoplastic material of the central layer 21 can alsocomprise fibres. The layers 11, 20 and 22 can by way of example be madefrom glass fibres co-mingled with polypropylene thermoplastic bindingfibres. The presence of the continuous fibres in the layer 11 of thefirst part (in this example a main body) and in at least one, butpreferably both of the layers 20 and 22 of the second part (in thisexample an end part) substantially increases the strength of theconnection and thereby of the vessel, even more so when both of thelayers 20 and 22 comprise substantially continuous fibres, and enablesalso to use, in particular for the thermoplastic layers 10, 12 and/or 21relatively inexpensive materials such polypropylene. Preferably thethermoplastic material substantially is polypropylene, PA, PET or PBT.When, within the framework of the invention mention is made of ‘layersof thermoplastic material with substantially continuous fibres’ or‘layers of thermoplastic material’ this does not exclude such a layerfrom comprising additional components, such as pigments, fillers,additional small (cut) fibres, anti-corrosion agents, protective inneror outer coatings, flame retardants, etc, etc. In this example the mainbody comprises layers 10 to 12 and the end part layers 20 to 22, withinthe framework of the invention fall also embodiments in which the endpart comprises layers 10 to 12 and the main body layers 20 to 22, andcentral layer 11 is at the seam sandwiched between outer layers 20 and22. The first part may also be one end part, while the second part isanother end part. It is preferred though that the pressure vesselcomprises a cylindrical main body, at one, preferably both edgesconnected to end parts. This enables a flexibility in the design, i.e.by choosing the length of the main body the volume of the pressurevessel can be freely varied, without having an influence on theconnection. In such embodiments both end parts are preferably of thesame design, which reduces the complexity of the design and productionmethod.

FIG. 1 also illustrates schematically the method in accordance with theinvention: the facing surfaces of the main body 1 and the end part 2 arepushed against each other, while the main body (or the end part) isrotated or more general moved in the direction schematically indicatedby an arrow in FIG. 1. The friction between the touching surfaces meltsthe thermoplastic materials forming a joint between the main body 1 andthe end part 2. Because of the continuous nature of the fibres a verystrong bond is achieved. This can be done for instance by spinning onepart and then letting it freely rotate and pushing said part against theother part. The friction between the two parts will on the one handcause the weld to be made, while on the other hand the spinning part isbrought to a halt. Preferably, however, one of the parts is activelydriven while being rotated. Such a method gives a much better controlover the friction welding process. During the friction welding the forceneeded for rotating one of the parts, or for keeping the other part inplace, may be measured and used as a parameter for controlling therotational speed and/or the relative movement of the parts, whichenables a better control over the welding process.

FIG. 2 illustrates a preferred embodiment of the pressure vessel andmethod of the invention. The contact area shows a protrusion 23 of thelayered structure of the main body into the central part 21 of thelayered structure of the end part. The protrusion increases the contactarea and, due to the protruding effect, increases the strength of theconnection. Preferably, as shown in FIG. 2, the protrusion is formed onthe main body. However, the roles could be reversed, i.e. the first partcould be the end part and the second part could be the main body. Inthis embodiment the layer 11 with the continuous fibres shows an overlapalong a length d with the layers 20 and 22 with continuous fibres. Thisoverlap adds strength to the vessel.

The layers 20 and 22 are shown here as not touching the layer 11 sincelayer 11 is smaller than layer 21. In embodiments, however, the layer 11could have a larger thickness than layer 21 and layers 11, 20 and 22could be touching each other.

A preferred embodiment of the invention is shown in FIG. 3. Herein thesecond part is provided with a slit 24, and the layer 11 of the firstpart is provided with a protrusion of comparable length to the slit.Preferably this protrusion is bevelled. The optional layers 10 and 12are indicated by dotted lines. Using such a bevel and slit enables abetter control over the welding process and a longer length d of theweld. In this example the inner layer 21 is shown recessed, however asmall layer of the thermoplastic material of layer 21 may be present atthe inner sides of layers 20 and 22. Preferably the length d is between10-100, preferably between 20 and 50 mm. Too small a length increasesthe risk that the weld or seam comprises weak spots, however too long anoverlap increases the risk that the innermost or outermost parts of theseam are not well connected, which also gives problems. A first step inthe welding method is preferably formed by inserting the protrusion intothe slit to find (and then record) the end (bottom) of the slit. Duringwelding the spinning can then be stopped approximately when the bottomof the slit is reached by the end of the protrusion; the spinning caneven be stopped only when the end of the protrusion has passed thebottom of the slit by a certain predetermined distance. The length ofthe substantially continuous fibres if preferably at least as long, morepreferably at least twice the length d.

FIG. 4 shows in more detail how the connection is made. The first partcomprises the layer 11, which is provided with substantially continuousfibres, schematically indicated by the criss-cross pattern. Thecontinuous fibres are preferably oriented in such manner, i.e. in acriss-cross manner, which can be achieved by winding the fibres andthermoplastic material on a mandrel in a criss-cross pattern and thenheating to fuse and/or consolidate the thermoplastic material. In thismanner a tube can be made, this tube may optionally be provided with aninner layer (liner) 12, and an outer layer (surface layer) 10, the tubeis then cut into segments, where after the edges of the segments arebevelled on a lathe.

The two parts are brought into position, close to each other, whereafter one or both of the parts is rotated (as indicated by the arrow)and the two parts are brought towards each other, during which movement,the bevelled end 23 enters the slit 24. Bevelling the edge increases thestrength of the weld, and in addition it decreases the risk that,immediately at the start of the welding, the edge of the first partdeforms the other edge of the second part. The bevelled shape of theprotrusion guides this part into the slit, even if there is smallmisalignment. The protrusion 23 and/or the slit 24 may be pre-treatedprior to welding. Pre-treatment may be constituted by the provision of athin layer of thermoplastic over the part 23 and/or inside the slit 24.

FIG. 5 illustrates a method for making a second part, in this example anend part 2 shaped as a cap. A layered structure is made comprising threelayers, a layer of thermoplastic material (the white layer in FIG. 5)sandwiched in between two layers of thermoplastic material withsubstantially continuous fibres (the layers depicted by a criss-crosspattern in the figure). In this example the central layer comprisespolypropylene within which cut glass fibres (having lengths of the orderof ten to 100 millimeters) are mixed, whereas the two outer layerscomprise fibre mats or fibre fabrics, for instance woven fabrics ofglass fibres of substantially continuous length, in a polypropylenematrix. One way of making such a layered structure is to make a stack ofa polypropylene film/a continuous glass fibre fabric/a polypropylenefilm with cut fibres/a continuous glass fibre fabric/polypropylene film,and then heating this stack to fuse the layers. Thereafter a (round)piece 2′ is cut from the thus made stack. This round piece is drawn overa shaping die, or compression moulded in a closed mould, to provide acap-shaped part 2. Some heat is applied during the deformation of 2′into 2. Thereafter the cap-shaped part 2 can be brought close to acircular shaped die 51 with an bevelled end 52, and the cap 2 is driveninto the die, forming a slit at the edge. Alternatively, the die 51 canbe integrated in the compression mould used to shape part 2, combiningthe shaping of the part and of the slit into one step. Alternatively theslit may be cut into the end cap. A combination of these methods, forinstance first forming a small slit with the die 51 and then making alarger or deeper slit by means of cutting, is also possible.

FIG. 6 illustrates a preferred embodiment of the invention. A body part1 and two end parts 2 are positioned in a lathe. The end parts 2 arespun synchronously in a lathe and brought towards part 1. Because theend parts 2 are spun synchronously they keep before, throughout andafter the welding process their mutual relative position. Alternatively,part 1 can be spun while the end parts 2 are not rotating. Both endparts are provided with handles 62 with apertures 63. Because the endcaps keep their mutual orientation, the handles also keep their mutualorientation. The friction weld is made.

In short the invention can be described by:

A plastic pressure vessel comprises a first (1, 2) and a second part(2,1) welded to each other. The first part (1) comprises a layer (11) ofthermoplastic material having substantially continuous fibres. Thesecond part comprises a layered structure having a first (20) and asecond (22) layer of thermoplastic material having substantiallycontinuous fibres separated by a central (21) layer comprisingthermoplastic material. The layer having substantially continuous fibres(11) of the first part extends over a length (d) sandwiched in betweenand connected with the first (20) and second (22) layer of thermoplasticmaterial having substantially continuous fibres of the second part.Preferably the second part is provided with a slit (24) and the firstpart with a fitting protrusion, preferably with a bevelled end (23). Inthe method the two parts are friction welded.

It will be apparent that the invention is not restricted to the abovegiven exemplary embodiments, but that within the scope of the inventionvariations are possible.

For instance, in the examples layer 11 is along a length interspersed(sandwiched between outer layers 20 and 22. So the sandwich can bedescribed by A (layer 20) B (layer 11) A (layer 22). These ‘outerlayers’ may themselves (one and/or the other) be covered with furtherlayers. All sandwiched layers comprise substantially continuous fibres.Preferably the layers 20 and 22 are similar or the same, because thissimplifies the structure and process, but they may differ in composition(for instance, if the outermost layer is to provide a different kind ofprotection than the innermost layers, or simply a different color), inwhich case the sandwich structure can be described as ABA′. Morecomplicated structures would be formed by providing two protrudinglayers sandwiched between two other layers (ABAB), two between threeABABA etc. The shown embodiment (of structure ABA, ABA′) is preferreddue to its simplicity and low costs.

Furthermore in the above disclosed embodiments the vessel comprises two(when comprised on two end caps) or three (when comprised of a body partand two end parts) parts. Although these embodiments are preferredembodiments from the point of view of simplicity of design, theinvention is not restricted to pressure vessels comprising two or threeparts, in embodiments the pressure vessels may comprise more than threeparts (for instance two body parts and two end parts). Such embodimentsmay for instance be useful if the pressure vessel is to be provided witha side entrance. The pressure vessel may then have a simple cylindricalbody part, a body part with a side entrance, which are connected to eachother by spinning the simple body part and connecting the two, andthereafter providing two end caps.

1. Plastic pressure vessel for a fluid comprising a first (1, 2) and asecond part (2,1) connected to each other, one of the first and secondpart being a main body (1) or an end part (2), characterized in that thefirst part (1) comprises at least one layer (11) of thermoplasticmaterial having substantially continuous fibres, and the second part (2)comprises a layered structure having a first (20) and a second (22)layer of thermoplastic material, at least one of said first and secondlayers (20,22) having substantially continuous fibres, said first andsecond layers (20, 22) being separated by a inner plastic layer (21),the layer (11) having substantially continuous fibres of the first part(1) extending over a length (d) sandwiched in between and connected withthe first and second layer (20, 22) of thermoplastic material of thesecond part (2).
 2. Plastic pressure vessel as claimed in claim 1,characterized in that the first and second layer (20, 22) of the secondpart (2) comprise substantially continuous fibres.
 3. Plastic pressurevessel as claimed in claim 1, characterized in that at least one of thefirst and second part is a cylindrically formed main body (1) of thepressure vessel, at both sides being provided with end parts (2). 4.Plastic pressure vessel as claimed in claim 1, 2, or 3, characterized inthat the second part comprises a slit into which a protrusion of thelayer comprising substantially continuous fibres of the first partextends.
 5. Plastic pressure vessel as claimed in claim 4, characterizedin that the protrusion is bevelled.
 6. Plastic pressure vessel asclaimed in claim 1, characterized in that the connection of the end partand the main body is formed such that the continuous fibres containinglayer of the first part extends, seen in a direction parallel to thelayers, over a length (d) between 10-100, preferably between 20 and 50mm in between the first and second layers of the second part.
 7. Plasticpressure vessel as claimed in claims 1, 2 or 6, characterized in thatthe length of the substantially continuous fibres in all relevant layersis at least equal, preferably at least twice the length (d) of overlapbetween the layers.
 8. Plastic pressure vessel as claimed in claim 1,characterized in that the substantially continuous fibres are laid inboth parts and all layers in a criss-cross pattern.
 9. Method formanufacturing a plastic pressure vessel as claimed in claim 1,characterized in that the first and second part are connected to eachother by friction welding.
 10. Method as claimed in claim 9,characterized in that a central part and two end parts, the central partbeing formed as a first part and the end parts as second parts, arefriction welded together, the end parts and/or the main body beingrotated.
 11. Method as claimed in claim 10, characterized in that theend parts are rotated synchronously.
 12. Method as claimed in claim 10or 11, characterized in that the end parts are provided with handles(62) prior to friction welding.